1. Field of the Invention
The present invention relates to a device for driving a laser.
2. Related Background Art
There is already known a printer for image recording with a semiconductor laser, which will be hereinafter simply called a laser. For stabilizing the amount of light emission from the laser, the Japanese Patent Laid-open No. 106259/198I discloses a stabilizing device in which the amount of light emission from the laser is detected in a non-image area and regulated to a determined value, and is maintained in the image area.
Reference is now made to FIG. 4 for explaining a conventional laser emission control method.
FIG. 4 is a chart showing the light emission characteristic of a laser, in which the ordinate represents the amount of light while the abscissa represents the current. Curves a, b show currents of different switching modes, and a curve c indicates the light emission.
At the image recording the current to the laser is switched, and the current is completely reduced to zero when the light is turned off. As shown in said chart, the laser emits light at a current equal to or exceeding a threshold current I.sub.th, and reaches a nominal amount of light L.sub.1 at a current I.sub.a. Thus the laser functions in a current range from I.sub.th to I.sub.a. Consequently a same light emission c is obtained with a current a or b.
However, when the current is varied from 0 to I.sub.th in a switching transistor for driving the laser, there results an increased loss therein, thus requiring the use of a transistor with a large loss tolerance. Also a transistor with a high switching speed is needed for high-speed image recording. Such a transistor with a large loss tolerance and capable of high-speed switching is very expensive.
Also there is required a considerable time before the laser emission can be started, due to the start-up time of the driving transistor for increasing the current from zero to I.sub.th.
In order to overcome to above-mentioned drawbacks there is proposed to supply to a laser with a bias current not exceeding I.sub.th. However the threshold current I.sub.th varies if the ambient temperature is changed, so that a fixed bias, applied to the laser, gives rise to an excessive current supplied to the laser, thus causing excessively strong emission if the ambient temperature is lowered.